Multiple access schemes in a conventional UWB communication system include Time Division Multiple Access (TDMA) for allocating time slots or time resources to calls by a management node, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) for attempting to make an access after finding out that a shared channel is available, Time Hopping Multiple Access (THMA), Code Division Multiple Access (CDMA), and so on. However, since these techniques had already been used broadly before a UWB communication technique were developed, they should be modified or improved properly in accordance with the recent physical layer technique.
TDMA is a technology that allows multiple accesses, in which a management node inquires of each node whether to transmit data and assigns a specific orthogonal time slot to a node that requests data transmission. Even though TDMA is useful for efficiently managing the service quality of each node, it is difficult and highly complex for a management node to manage all nodes every second for the operation of networks. Unfortunately, the complexity is sharply increased as the number of nodes increases.
CSMA/CS is a network control technology for sharing a channel in which a carrier sensing scheme is used to find out whether another node uses the channel before attempting to transmit data. When an ACK (acknowledgement) is not received or when more than one node uses the shared channel at the same time, a collision is announced and communication is tried again after a specific amount of time. Even though CSMA/CS can have a very simple embodiment, it has a difficulty in sensing whether the shared channel is being used particularly when the UWB signal power is very low and the effects of multiple paths fading are great. When the reliability on the detection possibility of channel usage is lowered, the performance of CSMA/CA is substantially deteriorated. In order to get high reliability, sensing needs to be done for an extended amount of time but this indicates the efficiency becomes low. Moreover, in case of CSMA/CA, when the number of nodes is greater than a certain level, the success probability of multiple access is lowered noticeably.
Recently developed multiple access schemes include THMA and CDMA. These schemes are robust against interference by averaging multiple user interference, and have excellent frequency domain properties of UWB signals. Nevertheless, when the distance from a management node to each node differs, they generate “near-far” effects and the amount of interference increases when the number of nodes becomes greater than a given level so that all nodes may not be able to communicate with each other. This problem occurs because signals of plural nodes accessing to the management node are not orthogonal to each other. In case of conventional multiple access schemes, it was impossible to use orthogonal codes because node signals were not synchronized with respect to the reception time of the management node.
FIG. 1 shows an operation of an asynchronous uplink in a conventional system. When the distances from a management node 305 to nodes are different, data transfer times 310, 315 and 320 from the nodes to the management node are also different. In addition, although the nodes transmit data at the same time, the management node receives the data at different times. Attempting to communicate using orthogonal time resources under this circumstance only causes interference to another node in the communication area. Therefore, instead of using orthogonal time resources, a pseudo noise (PN) code which is a resource of a low correlation may be used (DS-CDMA or THMA), or a shared channel may be checked and used (CSMA-CA), or a random channel may be used (ALOHA).
However, interference still exists in communications even if a PN code is used despite its low correlation. Such interference may cause a very serious problem to a system without power control. In case of CSMA in which a node verifies the absence of other traffic before transmitting on a shared channel, communication cannot be made unless the shared channel is absolutely clear or no other traffic is present. This corresponds to a case where multiple path fading is bad or a case having the problem of “Hidden node/Exposed node”. Lastly, using a random channel, such as in ALOHA, is not very efficient since the success probability of communication is noticeably lowered as the number of nodes increases.
Meanwhile, the frame structure for use in the access scheme in conventional UWB communication systems is a super-frame composed of an active period and an inactive period. The active period in which nodes are activated for communications consists of a beacon period, a contention access period, and a contention free period. On the other hand, in the inactive period all nodes of a network including a network coordinator are inactivated. In the contention free period, TDMA-based technology is used for a management node to operate the network, so it is difficult and highly complex for the management node to manage all nodes every second. In the contention access period, however, a CSMA/CA based technology is used, so it is difficult to sense the traffic on a shared channel when the power of a UWB signal is very low and the effects of multiple path fading are great. Especially, as the number of nodes increases, complexity of TDMA scheme in the contention free period increases sharply and the success probability of multiple access of CSMA/CA scheme in the contention access period is noticeably lowered. In consequence, the performance of the entire communication system is deteriorated.